Numerous studies have focused on high dietary fat intake and its association with chronic diseases in humans. It is well known that fatty acid peroxidation and fatty acid hydroperoxide decomposition are free radical-mediated processes. We report in vivo evidence for fatty acid-derived free radical metabolite formation in bile of rats dosed with spin traps and oxidized polyunsaturated fatty acids (PUFA). Fatty acid-derived radical adducts of a-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN) were detected in vivo in bile samples. Upon the administration of oxidized 13C-algal fatty acids and 4-POBN, the EPR spectrum of the radical adducts present in the bile exhibited hyperfine couplings due to 13C. Our data demonstrate that the carbon-centered radical adducts observed in in vivo experiments are unequivocally derived from oxidized PUFA. This in vivo evidence for PUFA-derived free radical formation supports the proposal that processes involving free radicals may be the molecular basis for the previously described cytotoxicity of dietary oxidized PUFA. Nitric oxide plays very important functions in the physiology of cardiovascular, immune and nervous systems. Our study suggests the possibility that spin traps, PBN or 4-POBN, may be sources of nitric oxide in biological environments. When utilizing these nitrones as spin traps in biological samples, not only is the trapping of reactive free radicals operative, but nitric oxide produced from the decomposition may play an important role in altering biological functions.